The invention relates to a signal transmission method of transmitting and receiving high rate signals between semiconductor integrated circuits, for example, and an output drive circuit for use therewith.
A multiplicity of semiconductor integrated circuits mounted on a circuit substrate are electrically interconnected together for practical use through a printed circuit formed on the circuit substrate in the art.
For transmission with preserved fidelity of signal waveforms as the signal being dealt with increases its frequency, there is a need that an associated signal transmission path has an evenly matched impedance. At this end, the signal transmission line generally comprises a microstrip line so that the transmission line is matched to a given characteristic impedance. It is also recognized as another requirement for achieving a transmission without a disruption of signal waveforms that a termination resistor having a resistance equal to the characteristic impedance of the signal transmission line be connected to either one, preferably both, of a receiving and a transmitting end in order to prevent a reflection from occurring at either end.
FIG. 7 shows an example where a balanced signal is transmitted from a driver DR of a semiconductor integrated circuit LSI.sub.1 to a receiving side semiconductor integrated circuit LSI.sub.2 through a balanced signal transmission path LIN, which is matched to a given characteristic of impedance Z.sub.0, and a termination resistor RT equal to the characteristic impedance Z.sub.0 is connected to the receiving end at the receiving semiconductor integrated circuit LSI.sub.2.
FIG. 8 shows another example in which the termination resistor RT is connected to the transmitting end. The fact that the connection of the termination resistor RT equal to the characteristic impedance Z.sub.0 of the signal transmission path LIN to either receiving or transmitting end allows a distortion in the received signal waveform which might result from reflections to be blocked is well known in the art, but for purpose of subsequent discussions, the basis therefor will be described briefly.
When the termination resistor RT is connected to the receiving end as shown in FIG. 7, the transmitted signal is absorbed by the termination resistor RT at the receiving end, producing no reflection. Accordingly, the received signal can be accepted by the receiving side free from any influence of reflections.
On the other hand, when the termination resistor RT is connected to the transmitting end, the transmitted signal is reflected by the receiving end and then travels reversely along the signal transmission path. Since the reflected wave is arithmetically added to the received signal, the amplitude of the received signal will be approximately doubled. However, there occurs no distortion in the waveform because the signals accepted at the receiving end are of the same timing. As the reflected wave travels backward on the signal transmission path LIN, it passes by the signal directed toward the receiving end, but does not impart any distortion to the latter, whereby the latter signal is transmitted to the receiving end while maintaining its proper waveform (even though the waveform may appear distorted as a result of a sum of signals which pass by each other if it is observed at a given point on the signal transmission path LIN). When the reflected wave returns to the transmitting end, it is absorbed by the termination resistor RT at the transmitting end to disappear.
It will be understood from the preceding description that the connection of the termination resistor RT to either the receiving or the transmitting end allows the influence of reflections to be eliminated and allows the transmission with no distortion in the waveform. It will be readily apparent that the termination resistor RT will be preferably connected to both the transmitting and the receiving end to eliminate the influence of reflections more completely.
It is essential to blocking signal reflections that the termination resistor RT be connected to either one or both of the receiving and the transmitting end. It is necessary that the resistance of the termination resistor RT exactly matches the characteristic impedance Z.sub.0 of the signal transmission path LIN. Accordingly, a high accuracy is required of the resistor being formed. It will thus be seen that it is not advisable to form the termination resistor RT within either semiconductor integrated circuit LSI.sub.1 or LSI.sub.2 since this leads to a cost increase disadvantageously. The termination resistor RT is commonly mounted outside the semiconductor integrated circuit LSI.sub.1 or LSI.sub.2. However, as the number of signal transmission paths LIN between the semiconductor integrated circuits LSI.sub.1 and LSI.sub.2 increases, the number of termination resistors RT required also increases, requiring an increased area for their mounting and presenting a difficulty to the achievement of a reduced size and a higher density of devices. The problem is even more remarkable when employing the super-high density mounting technique such as MCM (multi-chip-module), CSP (chip-size-package) or the like.